These words only add value to English because they are vague; if that were not the case, English would only need the cardinals to represent quantities. 

A vague term, by definition, has no discrete boundary between itself and its coordinate term (its semantic neighbour). The gradated boundaries of vague terms make them uniquely useful. Consider that a vague term conveys more information than a range of values conveys. A vague term can be conceived to be a set of continual (non-discrete) values; the lesser the difference between the value and the median, the greater the value's truth-hood will be. For instance, the word 'bald' becomes less true the less the subject's scalp resemble's Patrick Stewart's scalp. Defining a range of values, like 3-5, can be done concisely in words or mathematical notation, but it takes many more words and much more notation to precisely define a vague concept. So vague terms are semantically economical. 

In short, I think it's best to conceive terms such as 'several', 'couple', and 'few' to be overlapping value-ranges with no discrete boundary between any two of them.